I can see the use of DX if you’re going to crop anyway for larger viewfinder presentation…but for action it’s easier for me at least to keep the subject in the viewfinder in FX, although I need to do more me testing. My current thought on the Z7II is that perhaps putting the 500PF on it with perhaps a 1.4 TC and keeping it setup for really long shots might be attractive. Dunno…still thinking on that…and while I think in most cases having 2 bodies so no lens swap is needed the ‘it just works different’ is an issue. I’m unlikely to buy a second Z9 because I want a smaller body for less wildlife or photo centric trips…but one tha5 worked mostly like the Z9 would get me to trade the Z7II for it…if only for the consistency and if it was only 75% of a Z9 with lower FPS or whatever that’s a much closer match for me in how it works.There are two advantages of DX mode that cause me to seriously consider it some of the time. With mirrorless, you have a viewfinder that matches the actual image, so DX mode makes the central part of the image larger. That makes it a little easier to see subjects - particularly difficult subjects such as when they are obstructed or distant or distant in flight. The other advantage also relates to the EVF - the AF boxes are all bigger. With the Z9 you can choose Dynamic S,M or L, but you also can use DX which makes those boxes larger and might be helpful for some subjects. The same is true for the Wide S and L boxes - they are bigger in relation to the frame.
It can be hard using the Z7ii next to the Z9. The control layout and AF modes are quite different. I think it's an option if you need the Z7ii for something specific - like landscapes or environmental wildlife shots.